Recently, safety in electric cables has been highly desired, i.e., electric cables for atomic power plants, vehicles, various electronic devices, etc. have been desired to have excellent flame retardancy without the generation of toxic gas during combustion of the cables. In accordance with the above, heat-shrinkable tubes used for protection, insulation or the like of these electric cables have also been desired to have excellent flame retardancy without the generation of toxic gas during combustion of the tubes.
In order to attain the above requirements, methods for attaining flame retardancy of heat-shrinkable tubes have been proposed wherein a large amount of a metallic hydrate, such as aluminum hydroxide and magnesium hydroxide, is added to polyethylene or the like (as described in Japanese Patent Publication No. 41367/87); or the combination of metallic hydrate and red phosphorous or wherein the combination of magnesium hydroxide and carbon powder is utilized (as described in Japanese Patent Publication Nos. 13832/85 and 10898/82).
However, if an inorganic flame retardant such as metallic hydrate is added to a resin composition such as polyethylene in an amount capable of attaining excellent flame retardancy passing the Standard All-tubing Flame Test of Underwriters Laboratories (UL Standard), the mechanical properties of the resin composition, particularly the elongation, are deteriorated, such that the UL Standard (elongation of 200% or more) is not satisfied.
Moreover, in the methods using red phosphorous or carbon powder are not suitable for improving the flame retardancy of heat-shrinkable tubes which are required to be colored several different colors depending on their objective use, because the resin composition is colored red or black.